fix exception when loading user without submissions

[Tukajo]

This pull request is to fix an issue with users that do not have a "submissions" key.

If you look at the response for my username, https://hacker-news.firebaseio.com/v0/user/tukajo.json

It returns the following JSON

{"created":1650837729,"id":"tukajo","karma":1}

This causes an exception to be thrown when the getJsonArray is called, which causes the modal to never be able to render the user profile (no matter how many retries).

Addresses #10

[SimonHalvdansson]

This looks like a very good nice straightforward catch, thanks! :)